Pre-PR Checklist

• I linked an issue/discussion where a maintainer approved this enhancement/feature for implementation.

Approval Link

Closes #3435

Summary

This PR updates the Model class initialization to accept a Scenario class directly, automatically instantiating it with the appropriate default or provided rng. It maintains full backwards compatibility by continuing to support pre-instantiated Scenario objects or None.

Motive

The current pattern for wiring Scenarios into Models creates unnecessary boilerplate code in every model subclass (e.g., manually checking if scenario is None:). By allowing a Scenario class to be passed directly, developers can use the class as a default argument in their model's signature. This improves code readability, enables proper IDE autocomplete, and allows tools like SolaraViz or batch_run to easily inspect default parameters.

Implementation

1. mesa/model.py:

• Updated the Model.__init__ type hints to accept a Scenario class (scenario: S | type[S] | None = None).
• Modified the initialization logic to check if scenario is a class (isinstance(scenario, type) and issubclass(scenario, Scenario)). If so, it instantiates the scenario using the resolved rng.
• It continues to safely handle pre-existing Scenario instances and None fallbacks.

2. tests/test_model.py: Added a new test, test_model_scenario_initialization, which verifies the behavior when passing None, a pre-configured Scenario instance, and a custom Scenario class.

Usage Examples

Before:

class PdGrid(mesa.Model):
    def __init__(self, scenario=None):
        if scenario is None:
            scenario = PrisonersDilemmaScenario()
        super().__init__(scenario=scenario)

After:

class PdGrid(mesa.Model):
    def __init__(self, scenario=PrisonersDilemmaScenario, rng=None):
        super().__init__(scenario=scenario, rng=rng)
        
model = PdGrid()
model = PdGrid(rng=42)
model = PdGrid(scenario=PrisonersDilemmaScenario(width=100, rng=42))

Additional Notes

All existing tests and the newly added test_model_scenario_initialization pass locally.